1. Field of the Invention
The present invention relates, in general, to a frame transmission service in a multi-channel link and, more particularly, to an apparatus and method for preserving a frame sequence and distributing traffic in a multi-channel link, which efficiently distributes frame traffic while preserving the transmission sequence of frames in a link composed of multiple channels, and to a multi-channel transmitter using the apparatus and method.
2. Description of the Prior Art
Generally, a Wavelength Division Multiplexing (WDM)/Subcarrier Multiplexing (SCM)-Passive Optical Network (PON) based on WDM and SCM must support a multi-channel aggregation function so as to efficiently transmit variable-length frames through multiple channels.
For technology supporting a broad bandwidth required by a user using a digital communication service, there is Passive Optical Network (PON) technology. The PON is a subscriber distribution network for connecting communication channels required between a service provider's central office and a subscriber device to each other through optical wavelengths using optical passive elements. In order to couple a plurality of subscriber lines to a PON, a terminator connected to each subscriber side is designated as an Optical Network Unit (ONU), and a terminator connected to a service provider's central office side is designated as an Optical Line Terminal (OLT).
Fundamentally, since a plurality of ONUs is aggregated through an optical splitter in the PON, a plurality of subscribers shares a link from the optical splitter to a single OLT. Therefore, in an upstream service for transmitting from a subscriber side to a central office side, a multiplexing scheme for a shared link from the optical splitter to an OLT is required.
Research on WDM/SCM-PON establishing a link using WDM and SCM to perform communication required between an OLT and a plurality of ONUs has been actively conducted.
Such a WDM/SCM-PON uses an Arrayed Wavelength Grating (AWG) and an optical splitter/combiner, which are passive elements, to physically connect a plurality of ONUs. A link between the plurality of ONUs and the OLT is composed of a plurality of wavelengths and a plurality of subcarriers.
However, in the WDM/SCM-PON, a plurality of SCM channels coexists with respect to a single wavelength, so that a transmission band can extend by connecting ONUs to the OLT through the plurality of SCM channels in a point-to-point manner. In the case of a multi-channel link connected through the plurality of SCM channels in this way, technology preserving the sequence of frames at the time of transmission/reception is required. The Institute of Electrical and Electronics Engineers (IEEE) 802.3 link aggregation standard proposes a scheme of dividing traffic to be simply transmitted through a link into several conversations and preserving the frame sequence only within the conversations. Therefore, when the number of divisible conversations is small, or when bands between conversations are not uniformly configured, a link cannot be efficiently used.
In relation to this technology, several schemes of preserving the transmission sequence of frames have been proposed. For example, a padding method of transmitting a frame received from an upper system in 1523 octets, which is the maximum length available for Ethernet frame transmission, is disadvantageous in that it is very inefficient with respect to traffic including frames having short lengths, and in that a reception unit must extract a padding part.
Further, as another method, a division method of regularizing the length of frames by dividing the frames into sub-frames having regular lengths and by transmitting the sub-frames without padding the frames, is disadvantageous in that transmission/reception processing is complicated because each frame to be transmitted is divided into small sub-frames having regular lengths, a header including new division information is added to the sub-frames, the sub-frames having the header are transmitted, and a receiving side reconstructs the sub-frames on the basis of the header information. Further, since this division method cannot maintain compatibility with a lower layer (that is, WDM/SCM-PHY), a new physical layer is required.
As a further method, a tagging method of indicating a transmission sequence on frames is a method of writing the transmission sequence on a header, tagging the header to frames, and transmitting the frames tagged with the header. This tagging method is problematic in that the frames must be reconstructed with reference to the header information of received frames, and is disadvantageous in that the implementation of the method is highly complicated, for example, it requires the revision of standard protocols.
Therefore, it is impossible to efficiently use resources using only the conventional methods when a link is implemented using multiple channels as in the case of the WDM/SCM-PON.
In the meantime, Korean Patent Laid-Open Publication No. 2004-0074680, entitled “Method of distributing frame traffic in link aggregation Ethernet that balances and transmits frames through a plurality of links between applications”, discloses a method of comparing a currently received frame with a previous frame, and distributing the current frame to the same buffer when the current frame is smaller than the previous frame or when it does not contain length information. This method differs from the present invention of distributing traffic on the basis of a time at which a transmission service terminates using an actual measured frame length.
Further, U.S. Pat. No. 6,512,742 B1, entitled “System for data transmission across a link aggregation”, discloses a method of transmitting the same flow through the same link to preserve the frame sequence with respect to the flow, and determining transmission links of different flows in terms of load balancing without preserving a frame sequence with respect to different flows. This method differs from the present invention which primarily considers load balancing to select a channel and preserves the sequence of frames, entailing the possibility of a reorder problem occurring.
Moreover, Korean Patent Laid-Open Publication No. 2003-0033407, entitled “Packet realignment method”, proposes a method of realigning frames on a receiving side even if the sequence of frames is inverted during a frame transmission process. However, this patent differs from the present invention which provides a method of fundamentally preventing the inversion of a frame sequence.